Because advertisers like it when you duplicate work. :p
There are actually like 4 wiki's btw, the Wikia one, the fextralife one, and there are two more run by "corporate" game sites.. google you will find em. There may even more more then four..
It's too bad the gaming community doesn't have a "wikipedia" type site that ends up being the default go to for all such things.. but again, people want their ad revenue.